If a steel blankwork containing more than 1% Si is hot rolled, generally the hot rolled sheet is recrystallized at the surface layer only, and the middle layer is composed of a rolled and non-recrystallized structure. If such a hot rolled sheet is cold rolled and annealed as it is, magnetic properties could not be provided, since a texture conducive to the magnetic properties develops insufficiently. For securing the magnetic properties after the cold rolling and annealing, the hot rolled structure should be perfectly recrystallized. For example, Japanese Patent Application Laid Open Specifications No. 68717/79 or No. 97426/80, aiming at such objects, disclose annealings of the hot rolled sheet by a batch annealing or a continuous annealing after hot rolling and coiling.
In the annealing of the hot rolled sheet as such, if the recrystallization treatment is carried out on the hot rolled sheet, scales remain on the surface thereof, and if the annealing is done in an insufficient non-oxidizing atmosphere, the scales develop and grow thick, and internal oxidized layers grow in the steel surface layer so that a pickling ability after the treatment is markedly deteriorated. On the other hand, in spite of the non-oxidizing atmosphere, if the annealing is done in an atmosphere containing nitrogen, a nitriding reaction is accelerated in the steel surface layer, and it combines Al in the steel and brings about precipitations of AlN in the steel surface layer. Therefore, AlN particles considerably lower ferrite grain growth in a final annealing. As a result, the steel surface layer is formed with regions of fine ferrite grains of about 20 .mu.m in thickness of about 100 .mu.m, and remarkably deteriorate properties of iron losses and magnetic properties in low magnetic fields.
In view of these circumstances, Japanese Patent Application Laid Open Specification No. 35627/82 discloses the art of performing the pickling after the coiling at high temperature and subsequently batch annealing. However, at coiling temperatures of higher than 700.degree. C., not only does the scale on the surface grow thick, but also oxidation is caused in the ferrite grains, if Si is more than 1 wt %. The oxidized layer in the ferrite grain cannot be perfectly removed by the pickling before the annealing of the hot rolled sheet, and the magnetic properties are deteriorated as stated above.
Further, in the annealing of the hot rolled sheet, it is necessary to perfectly precipitate AlN for satisfactory ferrite grain growth at a final annealing, and coarsen the precipitated AlN for which a soaking time should be taken sufficiently in the annealing. If the soaking time is short and the coasening of AlN particles is insufficient, the grain growth at the final annealing is spoiled by the inhibiting effect of movements of the grain boundaries due to AlN particles.